The present invention relates to improved method and devices for measuring the electrical resistivity of geological formations traversed by a borehole.
At the present time, sondes for measuring the electrical resistivity of these formations emit into the formations a main electric current from a central electrode. A predetermined concentration or focussing of the lines of flow of the main electric current is obtained by means of an auxiliary current emitted into the formations by two guard electrodes on each side of the central electrode. The values measured with prior art sondes are average values which do not always represent with sufficient accuracy the sudden variations in resistivity of the geological layers adjacent a borehole drilled therethrough (e.g. French Pat. No. 1 075 314 and U.S. Pat. No. 3,262,050).